1991 Suburban bucking on light acceleration

Discussion in 'GM Powertrain' started by Crawdaddy, Apr 14, 2013.

  1. Crawdaddy

    Crawdaddy All hail the Mad King!! Staff Member 5+ Years 1000 Posts Platinum Contributor

    Yesterday I took the Suburban out, and it started to buck on me. When I would take off from a light with light acceleration, the truck would buck from around 1500 rpm up to 2500 or so. It would do that in virtually all gears. While the truck was bucking, the tachometer seemed to jump around a lot. At one point, I got a OBD-I code 42 which is related to the ignition module, but the light went out once I shut the truck off and turned it back on, but it still was bucking.

    I got home and had someone else take a ride with me to try to troubleshoot the issue and when it happened once, the engine seemed to make a clunking/rattling type sound. He seems to agree it's something ignition related. I initially thought it might be transmission-related, but it doesn't make sense that the other symptoms would be present with transmission issues. Any idea on what the issue could be?

    If it's ignition, I plan to change the ignition module with a genuine GM part (the existing one is an Autzone part) and upgrade the rest of the ignition while I'm at it with a hotter coil, high quality MSD distributor cap/rotor/wires, and change the plugs to factory plugs. I'll also likely increase the spark gap and advance the timing once I have confidence that I've solved this issue in an effort to get more power and fuel mileage.

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